Gas-fired, condensing water heater manufacturers, continuously strive to increase the efficiency of the water heater, that is to say the percentage of the combustion energy which is transferred to the water being heated in the tank. Such water heaters produce condensation and the heat released by the water vapor is relatively high and a large amount of energy of combustion is contained therein. By condensing this hot water vapor to exchange its heat with the water, the thermal efficiency of the water heater can be increased in the order of at least 90%.
One method of increasing the efficiency has been to capture the hot gases leaving the flue of the water heater and recycling these in an external heat exchanger to heat water to be fed back to the tank of the water heater or to use it for other purposes.
More recently some fabricators have constructed water heaters wherein the flue is extended to make a second pass into the tank for heat exchange with the water being heated and thereby increasing the efficiency. An example of such design can be found in U.S. Pat. No. 7,836,856 wherein it is stated that 80% of the heat from the combustion gases is transferred to the water within the water tank through the wall of the central flue without condensation, and an additional 10% or more of the combustion heat is transferred to the water accompanied by condensation of water in a coiled flue section which is welded to the top end of the central flue which is closed. The coil is supported by connecting rods which are welded to the base and the coil whereby to dispose the coil about the central flue at a downward angle to evacuate the condensation liquid. The bottom end of the coil exits the tank through a hole and welded thereabout. The installation of such an assembly is difficult and different type metal come into contact inside the tank and this would result in early deterioration of the sacrificial anode which is mounted in the tank to prevent corrosion of the submerged exposed metal leading to lower tank life.
Another example of such designs is disclosed in U.S. Pat. No. 7,992,526 wherein the central flue exits the top wall of the tank and the hot gases are fed back by a blower mounted on the top wall, into a second pass straight flue conduit, of fairly large diameter, extending through the top wall and extending into the tank to exit the tank at a lower side end thereof. Both flue conduits can be fitted with baffles to increase its efficiency and such baffles are well known in the art. The installation of the second pass flue requires the cutting of large holes in the tank wall to position the top and bottom end of the second pass flue and thereafter welds have to be made to seal the gaps between the second pass flue and the tank wall. Because of the large diameter of the second pass flue, the size of the holes need to be fairly large and of oblong shape to permit the flue ends to be fished there through resulting in difficult weld seams which could develop leaks and these welds create a different metal exposure to the inside of the tank and thus increasing further deterioration of the sacrificial anode causing it to be consumed earlier which results in the formation of corrosion inside the tank and a reduction in the life span of the water heater. The assembly of such water heaters is also difficult and more costly although they yield an efficiency exceeding 90%.